The number and type of atmospheric pressure techniques suitable for sampling analytes from surfaces, forming ions from these analytes, and subsequently transporting these ions into vacuum for interrogation by MS have rapidly expanded over the last several years. Moreover, the literature in this area is complicated by an explosion in acronyms for these techniques, many of which provide no information relating to the chemical or physical processes involved. In this tutorial article, we sort this vast array of techniques into relatively few categories on the basis of the approaches used for surface sampling and ionization. For each technique, we explain, as best known, many of the underlying principles of operation, describe representative applications, and in some cases, discuss needed research or advancements and attempt to forecast their future analytical utility.

The aim of this study was to identify antioxidants from Garcinia buchananii bark extract using hydrogen peroxide scavenging and oxygen radical absorbance capacity (ORAC) assays. LC-MS/MS analysis, 1D- and 2D-NMR, and circular dichroism (CD) spectroscopy led to the unequivocal identification of the major antioxidative molecules as a series of three 3,8″-linked biflavanones and two flavanone-C-glycosides. Besides the previously reported (2R,3R,2″R,3″R)-naringenin-C-3/C-8″ dihydroquercetin linked biflavanone (GB-2; 4) and (2R,3S,2″R,3″R)-manniflavanone (3), whose stereochemistry has been revised, the antioxidants identified for the first time in Garcinia buchananii were (2R,3R)-taxifolin-6-C-β-D-glucopyranoside (1), (2R,3R)-aromadendrin-6-C-β-D-glucopyranoside (2), and the new compound (2R,3S,2″S)-buchananiflavanone (5). The H₂O₂ scavenging and the ORAC assays demonstrated that these natural products have an extraordinarily high antioxidative power, especially (2R,3S,2″R,3″R)-manniflavanone (3) and GB-2 (4), with EC₅₀ values of 2.8 and 2.2 μM, respectively, and 13.73 and 12.10 μmol TE/ μmol. These findings demonstrate that G. buchananii bark extract is a rich natural source of antioxidants.

The pH dependence of uranyl(VI) complexation by citric acid was investigated using Raman and attenuated total reflection FTIR spectroscopies and electrospray ionization mass spectrometry. pH-dependent changes in the nu(s)(UO(2)) envelope indicate that three major UO(2)(2+)-citrate complexes with progressively increasing U=O bond lengths are present over a range of pH from 2.0 to 9.5. The first species, which is the predominant form of uranyl(VI) from pH 3.0 to 5.0, contains two UO(2)(2+) groups in spectroscopically equivalent coordination environments and corresponds to the [(UO(2))(2)Cit(2)](2)(-) complex known to exist in this pH range. At pH values >6.5, [(UO(2))(2)Cit(2)](2)(-) undergoes an interconversion to form [(UO(2))(3)Cit(3)](3)(-) and (UO(2))(3)Cit(2). ESI-MS studies on solutions of varying uranyl(VI)/citrate ratios, pH, and solution counteranion were successfully used to confirm complex stoichiometries. Uranyl and citrate concentrations investigated ranged from 0.50 to 50 mM.

The analysis of analytes deposited on, separated on, or otherwise distributed about a planar surface using desorption electrospray ionization mass spectrometry in a surface scanning sampling mode was investigated. The physical regions of the surface-impacting solvent/gas jet desorption/ionization plume were described. Under the conditions typical for desorption electrospray ionization used here, the impact plume formed an elliptical desorption/ionization region on the surface. Most effective desorption/ionization was obtained from a smaller elliptical area within the larger impact region that was centered on a point on-axis from the sprayer tip to the surface. Maximum signal from a given amount of material on a surface was observed with proper plume and sample alignment when the diameter of the sample spot was less than the diameter of the central high-efficiency desorption/ionization region of the impact plume. Solvent and gas flow out of this high-efficiency desorption/ionization region was found to limit analyte accessibility to this region via a "washing effect" when analytes were on smooth surfaces or on surfaces for which the analyte had little affinity. As such, the direction of surface scanning and scan speed during an analysis was found to be important for maximizing signal levels and signal reproducibility on particular surface types. Overall, the results presented illustrate means to improve analysis of sample spots on various types of surfaces using desorption electrospray ionization mass spectrometry in a surface scanning mode.

During the analysis of surface-spotted analytes using desorption electrospray ionization-mass spectrometry (DESI-MS), abundant ions are sometimes observed that appear to be the result of oxygen addition reactions. In this investigation, the effect of sample aging, the ambient lab environment, spray voltage, analyte surface concentration, and surface type on this oxidative modification of spotted analytes, exemplified by tamoxifen and reserpine, during analysis by DESI-MS was studied. Simple exposure of the samples to air and to ambient lighting increased the extent of oxidation. Increased spray voltage also led to increased analyte oxidation, possibly as a result of oxidative species formed electrochemically at the emitter electrode or in the gas phase by discharge processes. These oxidative species are carried by the spray and impinge on and react with the sampled analyte during desorption/ionization. The relative abundance of oxidized species was more significant for the analysis of deposited analyte having a relatively low surface concentration. Increasing the spray solvent flow rate and the addition of hydroquinone as a redox buffer to the spray solvent were found to decrease, but not entirely eliminate, analyte oxidation during analysis. The major parameters that both minimize and maximize analyte oxidation were identified, and DESI-MS operational recommendations to avoid these unwanted reactions are suggested.